The invention relates to a moistening nozzle of a paper web comprising a frame to which air and water is conducted, a water nozzle which is arranged inside the frame and wherewith water is conducted to the outlet of the moistening nozzle and an air nozzle wherewith air is conducted to the outlet of the moistening nozzle, and in connection with the air nozzle there are threads that bring the air into swirling motion and the air nozzle and the water nozzle are arranged one within the other to allow the air and the water to produce water mist that is sprayed out from the moistening nozzle.
The paper web is moistened to control the moisture profile of the paper web, for instance. Moistening can also be employed to control the curling of the paper web on the paper machine. Further, in online calendering, for instance, the management of given quality parameters requires moistening of good quality with water.
The paper web is moistened with a moistening apparatus that comprises a plurality of moistening nozzles in the cross direction of the paper web, which nozzles are used for blowing water mist containing air and water onto the surface of the paper web. DE 952 765 discloses a moistening nozzle. Said moistening nozzle comprises an air nozzle and a water nozzle arranged nested therein. The air nozzle comprises an external thread that brings the air into swirling motion. The swirling air that flows out captures the water from the periphery to form small particles. Said moistening nozzle provides a hollow cone spray pattern. The air and water nozzle are arranged inside the frame of the moistening nozzle with a mutual threaded joint in as centralized a manner as possible so that a gap between them is as even as possible. Due to the securing manner it is difficult to achieve or maintain centricity and evenness of the gap and consequently the spray pattern differs easily from the desired shape. All in all, the structure of the moistening nozzle is relatively difficult in the respect that it is very difficult to make moistening nozzles providing homogeneous sprays.
FI publication 91 366 discloses a moistening nozzle for a paper web, which also comprises a water nozzle in the middle of the moistening nozzle frame and an air nozzle arranged outside the water nozzle. In connection with the air nozzle there is a spiral piece with an external thread to bring the air flow into swirling motion. The water nozzle and the air nozzle are mutually arranged to secure one another. In addition, between the air nozzle and the water nozzle there is provided a ring-shaped gap whose width in the circumferential direction remains unchanged. The structure of this moistening nozzle is relatively complicated and therefore the moistening nozzle is difficult to manufacture. All in all, it is relatively difficult to center the air nozzle and the water nozzle in the moistening nozzle, and consequently it is very difficult to manufacture moistening nozzles that provide mutually homogeneous sprays.